The omop_domain table includes a list of OMOP-defined Domains the Concepts of the Standardized Vocabularies can belong to. A Domain defines the set of allowable Concepts for the standardized fields in the CDM tables. For example, the “Condition” Domain contains Concepts that describe a condition of a patient, and these Concepts can only be stored in the condition_concept_id field of the omop_condition_occurrence and omop_condition_era tables.
Further details can be found in the OMOP CDM Vocabulary Tables specifications - https://ohdsi.github.io/CommonDataModel/cdm53.html#Vocabulary_Tables

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